1. Field of the Invention
The present invention relates to a high dielectric constant ceramic material, and a method of manufacturing the same, and more specifically to a high dielectric constant ceramic material having a small temperature dependence of dielectric constant over a wide temperature range in particular, and the method of manufacturing the same.
2. Description of the Prior Art
As characteristic items required for dielectric material, there are high dielectric constant, low dielectric constant vs. temperature dependence, low dielectric loss (dissipation factor) small bias voltage dependence of dielectric constant, high capacitance-resistance product, etc.
Among these items, in particular, a sufficiently high capacitance-resistance product (CR product) is required. For instance, Standards of the Electronic Industries Association of Japan prescribes a CR product as high as 500 Mohm..mu.F at room temperature in the Standards RC-3698B entitled Multilayer Ceramic Chip Capacitors for Electronic Applications. Further, MIL-C-55681 prescribes high CR product values even at a high temperature such as 125.degree. C.
Furthermore, stable temperature characteristics (dependence) are required over a wide temperature range. For instance, capacitance change is prescribed as .+-.15% or less within a temperature range of -55.degree. to +125.degree. C. in X7R characteristics in the Standards of Electronic Industries Association (EIA), U.S.A., as shown in FIG. 1.
In the case of multilayer-type elements, since the internal electrode layers and the dielectic layers are sintered simultaneously (cofired) in the form as shown in FIG. 2, it is necessary to use an electrode material stable at the sintering temperature of the dielectric material. Therefore, when the sintering temperature of the dielectric material is high, expensive electrode materials such as Pt, Pd, etc. should be used. In other words, there exists a need for a dielectric material sinterable at a relatively low temperature of 1100.degree. C. or less so that low-cost material such as Ag is usable as internal electrode material.
In the conventional high dielectric constant ceramic materials, barium titanate (BaTiO.sub.3) is generally used as a base material, and stannate, zirconate, titanate, etc. are solid-soluted thereinto.
However, since the sintering temperature of the barium titanate based material is as high as 1300.degree. C. to 1400.degree. C., high-temperature resistant, high-cost material such as platinum or paradium should be used for the internal electrodes, thus resulting in increased cost.
To improve the above-mentioned disadvantages involved in barium titanate, various compositions have been developed. For instance, Japanese Published Unexamined (Kokai) Pat. (referred to as JPUP) No. 57-57204 discloses a ceramic dielectric composition mainly composed of lead iron niobate; JPUP No. 55-51759 discloses one mainly composed of lead magnesium niobate; JPUP. No. 55-144609 discloses one mainly composed of lead magnesium tungstate; and JPUP No. 58-217462 discloses one mainly composed of lead magnesium/iron/tungstate.
However, no high dielectric constant ceramic material has so far been obtained such that the dielectric constant is high; the temperature dependence is small over a wide temperature range as -55.degree. to +125.degree. C.; and the insulation resistance is high; that is, the various electric characteristics are excellent and further low temperature sintering is enabled.
On the other hand, some research has been done to obtain flat temperature characteristics by mixing compositions having different dielectric constant temperature characteristics. For instance, JPUP No. 59-203759 discloses a mixture of Pb(Mg.sub.1/3 Nb.sub.2/3)O.sub.3 -Pb(Mn.sub.1/2 W.sub.1/2)O.sub.3 based material, and Pb(Mg.sub.1/3 Nb.sub.2/3)O.sub.3 -PbTiO.sub.3 -Pb(Fe.sub.2/3 W.sub.1/3)O.sub.3 based material. However, this mixture has a large temperature coefficient of dielectric constant (T.C.C.) and insufficient temperature characteristics. Further, Japanese Journal of Applied Physics, vol. 24 (1985) Supplement 24-2, pp. 427-429 discloses a mixture of Pb(Fe.sub.1/2 Nb.sub.1/2)O.sub.3 and Pb(Fe.sub.2/3 W.sub.1/3)O.sub.3. However, the CR product which is important for a capacitor material is not considered; the T.C.C. thereof is high and the temperature characteristics are insufficient.